1. Technical Field
The presently disclosed subject matter relates to a method for manufacturing a hot cathode fluorescent lamp.
2. Description of the Related Art
Hot cathode fluorescent lamps have a filament coated with an emissive material (being a so-called “emitter”) in the form of carbonate. If such a filament is supplied with a current while under vacuum, heat energy is generated at the filament, thereby changing the emitter that is in the form of carbonate into the corresponding metal oxide (being activated) to exhibit an electron emission characteristic.
One conventional exemplary configuration of such a hot cathode fluorescent lamp is shown in FIG. 1. The hot cathode fluorescent lamp has mounts 54 and a glass bulb 55. The mount 54 is formed of a flare stem 52 and an exhaust pipe 53. The flare stem 52 seals a pair of lead wires 51 thereinside, and the lead wires 51 are connected to a filament coil 50. The mount 54 configured as described above is disposed in an end region of the glass bulb 55 using the flare stem 52. The inside of the glass bulb 55 is vacuumed through the exhaust pipe 53, and thereafter, the filament coil 50 is supplied with a current through the lead wires 51 to activate an emitter coated on the filament coil 50.
Such a conventional hot cathode fluorescent lamp should have an insulating coating on the lead wires 51 that extend from the flare stem 52 to the vicinity of the filament coil 50. This insulating coating can restrict the injection of electrons into the lead wires 51 located at a position which is opposite to the discharge passage. This restriction can reduce the electrode fall voltage and can suppress the voltage drop. In addition to this, it is possible to improve its luminous efficiency.
In some other hot cathode fluorescent lamps, the same effects can be given by using a bead stem instead of such a flare stem 52 (see, for example, Japanese Patent Application Laid-Open No. Hei 06-349448).
In the hot cathode fluorescent lamp configured as described above, the flare stem 52 seals the lead wires 51 and the exhaust pipe 53 therein. The lead wires are disposed substantially parallel with each other in the longitudinal direction of the glass bulb 55. It should be noted that the exhaust pipe 53 extends from the inside of the glass bulb 55 to the outside of the glass bulb 55. In addition to this, the lead wires 51 are connected to the filament coil 50 disposed in the end region of the glass bulb 55 and extend to the outside of the glass bulb 55.
In this instance, if the outer diameter of the glass bulb 55 is 7 mmφ, the outer diameter of the exhaust pipe 53 should be 2 mmφ (i.e., a very thin pipe), which is the minimum limit for fabrication, due to the positional relationship between the lead wires 51 and the exhaust pipe 53. Since the flare stem 52 must be formed by flame processing, it is difficult to use a larger-sized flare stem to ensure the dimensional accuracy. Therefore, the miniaturization of such a flare stem is limited. Accordingly, if the flare stem 52 is used for the mount 54, the outer diameter of the glass bulb 55, to which the flare stem 52 is to be attached, must be approximately 7 mmφ or greater. In other words, if a fluorescent lamp employs a glass bulb 55 with the diameter of less than approximately 7 mmφ, such a fluorescent lamp cannot employ a mount using the flare stem 52.
On the other hand, if a bead stem is used for the mount 54, one side of the glass bulb where the mount is located is utilized as an exhaust pipe section. In this instance, the lead wires are connected to the filament coil at respective ends and are positioned within the exhaust pipe section at respective other ends. In other words, the lead wires are positioned within the vacuum system to be in vacuum.
After the inside of the glass bulb is evacuated, the filament coil supported within the glass bulb is supplied with a current to activate the emitter coated on the filament coil. In order to connect the lead wires located inside the glass bulb with an external power source line, a clamp section to connect them should be provided inside the exhaust pipe. Accordingly, the clamp section should have an air discharge function as well as a chucking function for supplying a current. In order to achieve both of these functions, the clamp section is required to have an accurate and complex structure for keeping airtightness.
Furthermore, suppose that the lead wires are connected to the filament coil at respective ends and protrude from the end of the exhaust pipe section of the vacuum system at respective other ends. In this case, if the outer diameter of the exhaust pipe section (glass bulb) is less than approximately 7 mmφ, the outer diameter of the lead wires should be 0.3 mmφ or less, which is very thin in this type of lead wire. Accordingly, if the lead wires extend over a long distance, the wires may sag and/or bend undesirably, resulting in possible contact with each other or other problems.
Furthermore, if the diameter of the glass bulb is made smaller, the filament coil would be closer to the inner wall of the glass bulb. In this case, only with the bead stem, it is difficult to secure a certain gap between the filament coil and the inner wall of the glass bulb with high accuracy. In an extreme case, it would be conceivable that the filament coil is brought into contact with the inner wall of the glass bulb. If the filament coil comes into contact with the inner wall of the glass bulb, the heat generated at the filament coil may transfer to the glass bulb, resulting in a deterioration of the stable activation of the emitter. This may lead to unstable luminous intensity at the time of turning on. Furthermore, this may undesirably affect the product life characteristics of the hot cathode fluorescent lamp itself.